Example 1 — hello-world: pure-soft @attr decorator device

[coretl]

Part of the 5-example tutorial sequence (#388). Step 1: the simplest possible controller — a pure-soft "hello world" using the @attr decorator (the PyTango one-decorated-getter feel). No hardware backend, so it runs with zero external processes — the honest first rung.

Scope

• Add src/fastcs/demo/hello_world.py: a tiny controller with one or two soft attributes via @attr/@attr_rw (no real IO — soft values), e.g. a greeting/counter. The point is to show the decorator spelling at its simplest.
• Unit tests.

Acceptance

uv run --locked tox -e pre-commit,type-checking,tests green.

ADR: 0018
Parent: #388
Blocked by: #397

[coretl]

Unblocked by #397 closing (#423 merged into refactor), so this is done in #425.

src/fastcs/demo/hello_world.py — a greeting read-write pair, a polled message derived from it, and a polled uptime carrying units/precision, so the module covers the whole @attr surface with no simulator, socket or external process behind it. Tests in tests/demo/test_hello_world.py.

pre-commit and type-checking green; pytest src tests --ignore=tests/benchmarking 447 passed, with only the 10 pre-existing p4p/socket-family failures this sandbox cannot run.

— overnight agent

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